A wireless Local Area Network is a wireless communication system with radios having relatively high throughput and short coverage ranges. Many wireless LANs are based on iterations of the IEEE 802.11 standard. Radio signals passing between a transmitter and a receiver in an indoor environment are reflected from many surfaces of objects in that environment. This results in the radio signal following many different paths between the transmitter and receiver. This phenomenon is called “multipath.”
When the coherence bandwidth of the RF channel is on the same order as the signal bandwidth of the signal, multipath in a radio system using most narrow-band or spread spectrum communication techniques results in interference at the receiver that must be addressed. This interference is a result of the radio receiver performing a vector addition of all the signals received from all of the various paths they follow between the transmitter and receiver. This vector addition can result in a very weak resultant signal (destructive interference) or a strong resultant signal (constructive interference).
Whether the resultant signal detected at the receiver is affected by destructive or constructive interference is a function of the relative positions of the transmitter, receiver, and all other objects that reflect the radio signal along paths between the transmitter and receiver. Because the spatial relationship between all these objects is the determining factor in the result of the vector addition of the received signals, moving the transmitter or receiver by a small amount (on the order of a wave length) will have a significant effect on the resultant signal.
For modulation methods based on modulating a single carrier, spatial diversity takes advantage of this characteristic (i.e., that moving one antenna a small distance can have a great effect on the resultant received signal), by separating two or more antennae by a wavelength or more and sampling the received signal at each antenna, before choosing one of the antennae to be used for reception. This spatial diversity technique uses antennae with patterns (coverage areas) that are typically similar and overlapping. If the patterns did not overlap, the effect of using the antennae for spatial diversity would be reduced. Recently, techniques other than single carrier modulation have been used for radio WLAN communication. Specifically, Orthogonal Frequency Division Multiplexing (OFDM) has been utilized. OFDM is a broad-band communication mechanism that addresses the multipath issue in the design of the modulated signal itself. Therefore, spatial diversity has diminished utility with this type of radio signal.
Despite the use of OFDM, the need remains for further optimizing signal reception between transmitters and receivers. For example, a need in the art exists for increasing the coverage area of the radios associated with access points to enable reductions in the number of access points required to adequately implement a wireless network environment. A need also exists for maintaining user performance, network efficiency, and data throughput under increased user load in a wireless network environment. Embodiments of the present invention substantially fulfill these needs.